This invention relates to a method for suppressing the formation of certain alkylated phenols with one or more alkyl groups by transalkylating the alkyl phenols. More particularly, this invention is directed to a method for transalkylating ortho- and para-alkylated phenols with other phenols in the presence of a metal oxide catalyst by recycling these phenols into the feed stream.
It is often desirable to dealkylate or transalkylate the alkylated phenols obtained from coal tars of coal liquifaction processes to provide more valuable products, such as phenol. In addition, it is often desirable to dealkylate or transalkylate 2,4,6-trimethylphenol (TMP), a co-product in the synthesis of 2,6-xylenol, to more useful alkylated phenols and phenol. Useful alkylated phenols include p-cresol, o-cresol, 2,6-xylenol, 2,4-dimethylphenol, 2,4,6-trimethylphenol and the like.
A number of methods of dealkylating alkylated phenols are known to the art. These methods include thermal dealkylation, thermal hydrodealkylation, and catalytic hydrodealkylation. Thermal dealkylation involves exposing alkylated phenols to high temperatures (about 800.degree. C.) to achieve thermal cracking of the alkylated phenol and yield phenol. However, this process is not selective and a substantial amount of dehydroxylation occurs, producing the less valuable benzene and alkyl-substituted benzene species. This process is shown more particularly by Daly in Journal of Catalysis 61, 528 (1980), the contents of which are incorporated herein by reference.
Thermal hydrodealkylation of alkylated phenols involves exposing the alkylated phenols to high temperatures in the presence of steam or hydrogen or both, as is shown by Daly in U.S. Pat. No. 4,230,895. This process also causes a significant amount of dehydroxylation, which is undesirable since dehydroxylation produces less valuable products.
Catalytic hydrodealkylation is typically more selective than the processes described above and causes less dehydroxylation. Daly describes a process in U.S. Pat. No. 4,230,896 wherein alkylated phenols are reacted with steam in the presence of a catalyst comprised of a hydrous carrier, a deactivation suppressor and at least one promoter. Catalysts included within those described by Daly include platinum and palladium on alumina and mixtures of palladium and chromium oxide on alumina. A catalytic hydrodealkylation process which reacts alkylated phenols with hydrogen is described by Bjornson in U.S. Pat. No. 4,191,844. This reaction takes place in the presence of a catalyst consisting essentially of magnesium oxide and a Group IIA metal oxide such as manganese oxide. Although, these catalytic hydrodealkylation processes are more selective and cause less dehydroxylation than thermal hydrodealkylation, there still remains room for improvement. For example, the percentage of alkylated phenol converted to a new material is very low (about 40%) in the process described in U.S. Pat. No. 4,230,896 and dehydroxylation is still significant, providing 5-30 weight % dehydroxylated products. When the alkylated phenols are reacted with hydrogen in the process described by Bjornson, the rate of dehydroxylation is also high producing large quantities of dehydroxylated products (up to 50 weight %) at high rates of dealkylation. In addition, these processes which utilize a catalyst to dealkylate alkylated phenols are handicapped by the short lifetime of the catalyst due to coking. The catalyst must be reactivated or regenerated periodically and a deactivation suppressant is often necessary.
The disproportion of highly alkylated phenols with phenol over a tungsten oxide promoted magnesium oxide catalyst is described by Leach in U.S. Pat. No. 4,125,736. The catalyst contained 0.5 to 15% tungsten oxide and the reaction was executed with 1 to 15% water resulting in o-cresol and p-cresol as the main products.
An acidic catalyst containing 75-100% aluminum oxide and 0-25% silica for the transmethylation of alkylated phenols in the presence of phenol or cresol is described by Talley in U.S. Pat. No. 3,417,149. Talley in U.S. Pat. No. 4,533,767 describes the steam dealkylation of ortho and or para alkylated phenols without the loss of a hydroxyl radical without phenol present in the feed stream. In U.S. Pat. No. 4,533,786, Talley describes a similar steam dealkylation as described in U.S. Pat. No. 4,533,767, however, using zinc oxide as the main component of the catalyst as opposed to magnesium oxide. In U.S. Pat. No. 4,560,810, Talley describes the ortho dealkylation alkylated hydroxyaromatic compounds using a combination of chromium oxide and one of the oxides of zinc, iron, magnesium, or manganese.
The suppression of the formation of highly alkylated phenols by a catalytic transalkylation process comprising this invention provides high conversion rates, generally on the order of 25 weight percent or higher. In addition, the catalyst lifetime is extended for particular embodiments of this invention so as to reduce the frequency of regeneration.